This invention relates to a method for disassembling and repairing a sodium-handling apparatus, which involves a preparatory step of disassembling and repairing the apparatus directly in the atmosphere while sodium is retained on the surfaces of the apparatus and a step of removing sodium retained on the surfaces of the apparatus after disassembling and repairing, when large-size apparatuses in a cooling system of a fast breeder reacter using sodium as a coolant are repaired.
A heat transfer system of a sodium-cooling fast breeder is comprised of a large scale sodium-handling apparatus such as an intermediate heat exchange, a main circulating pump and a steam generator, in addition to a nuclear reactor. Sodium will be hereinafter referred to "Na", and sodium-handling apparatus as "Na apparatus". These Na apparatus are so designed and fabricated as to have a satisfactory operating capability; but after operation for a definite period of time or when some disorder appears, the Na apparatus it is necessary to remove the Na apparatus from the heat transfer system and subject the Na apparatus to inspection, repairing or other operation. Since Na is a chemically active material, Na deposited on the Na apparatus is washed away in advance when such repairing is carried out. For example, a larger type Na apparatus is cleaned with steam, and a smaller type Na apparatus is cleaned with lower alcohols, and then these apparatus are washed sufficiently with washing water. However, steam cleaning and alcohol cleaning are not always possible, and sometimes the Na apparatus must be hermetically filled with an inert gas while Na is deposited on the Na apparatus. In any cleaning or washing procedure, Na deposited on the Na apparatus is finally converted to caustic soda (NaOH), when Na reacts with water. Even if Na is not washed away, Na reacts with oxygen in the atmosphere according to the following formula: EQU 4Na + O.sub.2 = 2Na.sub.2 O
and further, Na.sub.2 O reacts with water to form NaOH according to the following formula: EQU 2Na.sub.2 O + H.sub.2 O = 4NaOH + 2H.sub.2
when NaOH is brought in contact with materials of construction of the apparatus, NaOH causes stress corrosion cracking even at a relatively low temperature, for example, 200.degree. C, so long as a load is applied to the materials of construction. Stress corrosion cracking is promoted at higher temperature. In conventional Na washing using water, NaOH is very soluble in water, and consequently NaOH is filled even in very small recesses existing on the surfaces of the materials of construction. It is often experienced that NaOH, once filled in such recesses, is removed therefrom by water washing only with great difficulty. So long as even NaOH filled in the recesses on the surfaces of the materials of construction can directly contact a liquid Na, NaOH will dissolve in Na, and there will be no actual fear of remaining NaOH, but since washing by Na itself cannot be expected at parts in contact with a cover gas, there will be a considerable fear of remaining NaOH. When the Na apparatus is to be restarted after the repairing, it is pre-heated to 150.degree. to 250.degree. C at first, and then to about 500.degree. C during the operation. Therefore, when the Na apparatus is washed with water and there remains even a very small amount of NaOH on the surfaces of the materials of construction, stress corrosion cracking is inevitably brought about.